1. Field of the Invention
The present invention relates to an optical information recording medium for recording, reproducing and erasing information signals using optical means, and a method of fabricating the same. More particularly, it relates to a phase change optical disk employing a phase change of an optical active layer.
2. Description of the Related Art
A technique for recording information signals by using a physical or chemical change, which is locally induced by a laser beam irradiation, of an optical active layer formed on a substrate has been well-known. Such a technique is put to practical use, for example, as a data-filing optical disk device. Particularly, information signals can be rewritten easily by using a phase change material or a magneto-optical material as the optical active layer.
A rewritable phase change optical disk typically has a structure wherein a first heat-resistant protection layer (a dielectric layer), an optical active layer (a recording layer) and a second heat-resistant protection layer (another dielectric layer) are formed on a transparent disk substrate in this order. Furthermore, a reflection layer is often formed on the second heat-resistant protection layer.
In the phase change optical disk, a phase change in the state of the optical active layer is utilized for record/erasure operations of information signals, as described hereinafter.
When a portion of the optical active layer is melted by heating induced by a laser beam irradiation and then quenched, the optical active layer is locally converted into an amorphous state. Thereby, an information signal is recorded therein. On the other hand, in order to erase a recorded information signal, the optical active layer is melted by heating induced by a laser beam irradiation and then cooled slowly so as to crystallize.
Reproducing of recorded information signals is also conducted by a laser beam irradiation, which utilizes large differences of a light absorption index and a light reflection coefficient between a region of amorphous state (a recorded mark region) and a region of a crystal state (a non-recorded region) of the optical active layer.
The optical active layer having the above-described features is formed on a substrate having grooves for guiding a laser beam.
Practical record/erasure operations of information signals is carried out by irradiation of a laser beam with a half-width of about 1 .mu.m. In this case, the intensity of the laser beam is modulated so as to have a peak power intensity and a bias power intensity depending on an information signal to be recorded.
For example, when a laser beam modulated in amplitude as described above irradiates on a phase change optical disk while rotating, a portion of the optical active layer heated by the laser beam with the peak power intensity is quenched after being heated beyond the melting temperature. Thus, the material in the portion is converted into an amorphous state, irrespective of the phase state before the laser beam irradiation (i.e., wheather it was an amorphous state or a crystal state). As a result, the recorded mark region is formed and consequently an information signal is recorded therein.
On the other hand, a portion of the optical active layer irradiated by a laser beam with the bias power intensity is heated beyond the crystallization temperature of the material of the optical active layer. Thus, a recorded information signal is erased.
As explained in the above mechanism of record/erasure operations, the optical active layer of the phase change optical disk has to endure repeated temperature rises up to the melting temperature or the crystallization temperature or more by the laser beam irradiation. In order to achieve good performance properties even under such a severe situation, a first and a second heat-resistant protection layers are provided on both sides of the optical active layer.
The function of the first and the second heat-resistant protection layers are as follows: Firstly, thermal damage of the substrate surface, deformation or evaporation of the optical active layer due to repeated record/erasure operations are reduced. Secondly, optical characteristics of the phase change optical disk are optimized. For example, the difference between the optical characteristics before recording and those after recording is enlarged. Also, a light absorption index of the optical active layer is optimized. Thirdly, record/erasure properties of information signals can be optimized by selecting suitable materials for the heat-resistant protection layers, since the thermal conduction properties thereof has a large influence on the heating/cooling process of the optical active layer.
The material of the heat-resistant protection layer is required to have the following properties: high optical transparency, a high melting temperature, high mechanical strength, suitable thermal constants (a thermal conductivity and a specific heat), a suitable refractive index and chemical stability. The reasons are as follows.
For transmitting a laser beam to the optical active layer with a high efficiency, the heat-resistant protection layer is preferably transparent because the laser beam penetrates therethrough.
When the melting temperature of the heat-resistant protection layer is lower than a thermal transformation temperature of the optical active layer, the heat-resistant protection layer changes its state earlier than the optical active layer during the heating/cooling process of record/erasure operations. In order to prevent such a problem, the heat-resistant protection layer preferably has a high melting temperature.
In addition, preferably, the heat-resistant protection layer is mechanically strong enough to prevent cracking in the heating/cooling process.
The following trade-off relation exists as to thermal properties of the heat-resistant protection layer. When the heat-resistant protection layer has too large a thermal conductivity, the energy applied by a laser beam irradiation thereto is liable to dissipate easily. This means a poor energy efficiency and a laser beam with a large irradiation energy is required for heating the optical active layer beyond the melting temperature thereof. Consequently, in order to obtain a phase change optical disk with a high sensitivity by improving energy efficiency, the thermal conductivity of the heat-resistant protection layer is preferably small.
On the other hand, however, too small a thermal conductivity of the heat-resistant protection layer makes it difficult to rapidly convert the optical active layer into an amorphous state by-quenching, that is, to form a recorded mark region. In addition, there is another problem in that a temperature difference between the center portion and the peripheral portion of the recorded mark region becomes larger because of less heat transfer therebetween. Particularly, when a laser beam with a large line speed is used for record/erasure operations, the heating/cooling period must becomes short. Thus, the erasure ratio decreases.
A suitable refractive index is required for the following reason. In order to improve energy efficiency of a phase change of the optical active layer caused by a laser beam irradiation, it is necessary to prevent reflection of an incident laser beam at the interface between the heat-resistant protection layer and the optical active layer as much as possible. Preferably, reflection of the laser beam therebetween is desired to be reduced to zero. Since a refractive index of the typical optical active layer containing Te as a main component is about 4, it is necessary to have a refractive index of the heat-resistant protection layer set to 4 or less so as to satisfy the non-reflection condition. For example, Japanese Laid-Open Patent Publication No. 63-103453 discloses a calculated result that a preferable refractive index of the heat-resistant protection layer exists in a range between 2 and 3.
As the material which meets the above requirements, oxides, nitrides, carbides, fluorides and mixture thereof have been used, heretofore. On the other hand, U.S. Pat. No. 4,847,132 discloses using a mixture of chalcogenides and vitrification accelerating materials as the material for the heat-resistant protection layer. This mixture can be obtained by mixing chalcogenides (e.g. ZnS, ZnSe, etc.) which are essentially crystalline materials with substances which easily vitrifies (e.g. SiO.sub.2, GeO.sub.2, TeO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Y.sub.2 O.sub.3, etc.). By mixing, the vitrification accelerating material separates crystal grains of the chalcogenide so as to make the grains finer. As a result, the mixed material is converted into an amorphous state and the thermal conductivity thereof becomes small.
As described previously, when the thermal conductivity of the heat-resistant protection layer becomes small, heat generated at a certain portion in the optical active layer by a laser beam irradiation is less likely to dissipate into the surroundings through the heat-resistant protection layer. Accordingly, it becomes possible to induce a phase change by a smaller laser beam power so as to form a recorded mark region, which results in high sensitivity. In addition, since a smaller laser irradiation power is required for recording/erasing information signals in this case, the optical disk is less damaged thermally. Consequently, an endurable life in repeated record/erasure operations is improved.
However, the above-mentioned mixture of the chalcogenide and the vitrification accelerating material has a problem in that when the thermal conductivity is reduced by conversion into an amorphous state due to mixing, the refractive index is reduced simultaneously. As described previously, the refractive index of the heat-resistant protection layer influences transmitting efficiency of the irradiated laser beam to the optical active layer. Accordingly, both of the thermal conductivity and the refractive index of the heat-resistance protection layer must be optimized so as to attain good performance properties of the phase change optical disk.
On the other hand, Japanese Laid-Open Patent Publication No. 3-241539 teaches that the life characteristics in repeated record/erasure operations are improved by using a material of the second heat-resistant protection layer which has a different thermal conductivity from that of the first heat-resistant protection layer (e.g. oxides, nitrides, etc.).
In this case, however, when the second heat-resistant protection layer composed of oxides, nitrides or the like is formed in the same vacuum vessel as that used for forming the other layers, contamination of the layers is liable to be arisen, which brings bad affects on the physical and chemical properties of the layers. Therefore, a plurality of vacuum vessels must be used for forming different layers, which is disadvantage from the standpoint of manufacturing cost.